A cryogenic wind tunnel, in which liquid nitrogen is sprayed into the closed wind tunnel body to get low temperature and high pressure gas, is used to produce high Reynolds number flow in fluid dynamic measurement. In this case, if we want to use a hot wire anemometer to measure flow velocity, the calibration curve, i.e. the relation of output voltage and wind speed must be obtained. Under these low temperature and high pressure conditions we must use thick tungsten wire. The effect of heat conduction through both ends is also important. Several investigations have been conducted to determine the character of hot wire placed in atmospheric air flow. In this study, an analysis of the relation of output voltage and flow velocity of a constant temperature type cot wire anemometer by changing its length and diameter was made for various values of temperature and pressure of nitrogen gas using a small cryogenic wind tunnel. It was concluded that the nondimensional relation acquired in the atmospheric condition can be extended in the cryogenic condition.